1. Field of the Invention
The present invention relates to a drive device for a synchronous electric motor, in which windings of a stator coil are connected together to form a star connection.
2. Description of Related Art
For example, Japanese Patent 3223842 (corresponding to U.S. Pat. No. 6,320,775) discloses a drive device, which includes an inverter circuit and a control circuit. The inverter circuit includes six transistors (i.e., three upper arm transistors located adjacent to a cathode bus bar and three lower arm transistors located adjacent to an anode bus bar) and six diodes (i.e., three upper arm diodes located adjacent to the cathode bus bar and three lower arm diodes located adjacent to the anode bus bar). The control circuit controls the inverter circuit.
Specifically, a direct current (DC) power supply device is connected between a neutral point of the stator coil and the anode bus bar. A capacitor is connected between the anode bus bar the cathode bus bar.
The control circuit executes a switching operation of the six transistors, so that three-phase alternate currents are supplied to a three-phase alternate current synchronous electric motor (hereinafter, simply referred to as a three-phase AC synchronous motor) in response to a voltage difference between the cathode bus bar and the anode bus bar.
Here, when one of the lower arm transistors is turned on, the corresponding current flows to the stator coil, so that the magnetic energy is accumulated in the stator coil in response to the current.
When the lower arm transistor is turned off, the corresponding current flows from the stator coil to the capacitor through the corresponding upper arm diode and the cathode bus bar. Therefore, when the switching operation of the six transistors is executed, the three-phase alternate currents are supplied to the stator coil, and the electric charge is accumulated in the capacitor. In this way, the electric charge can be accumulated in the capacitor while controlling the operation of the three-phase AC synchronous motor.
In the case of the above drive device for the three-phase AC synchronous motor, when the output voltage of the power supply device is reduced due to, for example, environmental changes or load changes, the voltage difference between the cathode bus bar and the anode bus bar becomes small. When this happens, the operation of the three-phase AC synchronous electric motor becomes unstable, or the three-phase AC synchronous motor cannot be driven due to the output voltage shortage.
In view of the above disadvantage, it is conceivable to obtain the required voltage difference between the cathode bus bar and the anode bus bar by executing the switching operation of the lower arm transistors to accumulate the electric charge in the capacitor in response to the magnetic energy accumulated in the stator coil in the stop state where the control operation of the three-phase AC synchronous motor is stopped. However, in such a case, at the time of controlling the three-phase AC synchronous motor, the time period for executing the switching operation of the transistors may possibly be shortened.